In typical television receivers a radio frequency (RF) modulated television signal is received and processed, i.e., amplified, filtered, and demodulated to provide a composite video signal. The composite video signal contains video information which is utilized to modulate an electron beam or beams in a cathode ray tube and synchronizing information to synchronize the scanning of the electron beams of the cathode ray tube with the video information to create a coherent display. The synchronizing information is in the form of synchronizing pulses which extend beyond the black level of the composite video signal and which occur during the vertical and horizontal retrace or blanking intervals.
The synchronizing pulses are separated from the composite video signal in an amplitude clipper known as a synchronizing pulse separator. High level noise pulses included in the composite video signal may also contain sufficient energy to cause the synchronizing pulse separator to operate thereby deleteriously affecting the scanning of the cathode ray tube. Furthermore, typical synchronizing pulse separator circuits include a self-bias circuit which establishes the clipping level of the circuit. Noise pulses, especially continuous high energy noise, which reach the synchronizing pulse separator circuit affect the self-bias circuit to undesirably alter the clipping level thereby further deleteriously affecting the scanning of the cathode ray tube.
In view of these well-known deleterious effects of noise pulses, much effort has been directed to circuits and schemes for cancelling or suppressing the effect of high level noise pulses. Typical prior art techniques include noise gates which disable the synchronizing pulse separator in the presence of high level noise and noise cancellation circuits which clip and invert the noise pulse and add the inverted noise pulse to the composite video signal to cancel the noise pulse therefrom. Other similar techniques are also known in the prior art.
Typical television receivers also include automatic gain control circuitry which detects the amplitude of the synchronizing pulses and adjusts the gain of the signal receiver amplifiers in response thereto. A noise free composite video signal is also required for the automatic gain control circuitry so that high level noise pulses do not cause undesired gain changes. If the signal level changes abruptly, however, the noise cancellation circuit may cancel the synchronizing pulses from the video signal as well as cancel noise pulses thereby leading to a condition called system "hang-up" where the automatic gain control circuit increases the amplifier gain when the gain should be decreased.
While noise cancellation circuits using various prior art techniques have been developed and operate more or less satisfactorily, they suffer from one or more various disadvantages. For example, some prior art circuits deleteriously affect operation of the television receiver. Other circuits are unduly complex and/or require intricate control adjustment to avoid problems such as "hang-up" or similar deleterious effects. Other prior art circuits cancel noise only from the synchronizing pulse separator or use compromises which result in deleterious effects under some operating conditions or less than satisfactory performance. Still other prior art circuits or techniques do not readily lend themselves to fabrication in integrated circuit form. Another noise cancellation system is of a fixed bias nature which means the system clips any noise spikes above a predetermined voltage level. This type of system is limited in how close the noise clipping level can be set to the expected synchronization pulse level, this being due to component and environment variations which result in variations of one or more of these levels. This can result in synchronizing pulses being interpreted as noise pulses, especially after a channel having a strong signal is selected.